Steered-cone retrodirective antenna



March 10, 1970 R. T. ADAMS 3,500,417

STEERED-CONE RETRODIRECTIVE ANTENNA Filed May 25, 1965 FIG 2(a) 1N VENTOR. RMERT 7: ADAMS W 5 ATTOfi/VE rs.

nite States 3,500,417 STEERED-CONE RETRODIRECTIVE ANTENNA Robert T. Adams, Short Hills, N.J., assignor to Sichak Associates, Nutley, N.J., a corporation of New Jersey Filed May 25, 1965, Ser. No. 458,586 Int. Cl. H01q 1/28 U.S. Cl. 343-705 8 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a novel retrodirective antenna, and in particular, to one which is capable of employment as a satellite repeater which is passive in nature.

The primary function of a communication satellite is to provide reduced transmission loss between ground transmitting and receiving sites. It should do this with a minimum of associated problems such as jamming, overload, size, weight, operating life, reliability and cost.

Active satellite repeaters have the outstanding advantage of on board amplification with attendant decrease of return path signal-to-noise ratio. Except for Echo and West Ford, this factor has heretofore been decisive. These type satellites, however, are of a generally limited life and since maintenance is at the moment impractical, any minor malfunction could preclude further service. Further, active satellite repeaters can inherently sustain only a predetermined traffic load which, when exceeded, or when exposed to jamming, obviates its effectiveness.

Accordingly, it is the object of this invention to provide an antenna, adaptable for space use, which exhibits easily steerable high gain.

It is another object of this invention to accomplish the foregoing object with a minimum size, weight and cost repeater and with maximum operating life and reliability.

It is a further object of this invention to provide a passive satellite repeater with high antenna gain without restricting coverage of the earth or requiring close attitude control of the satellite vehicle.

It is a further object of this invention to provide a space repeater which inherently precludes jamming and interference and which permits multiple users in a plurality of combinations.

It is a still further object of this invention to provide a passive satellite repeater which matches the roundtrip transmission loss of active satellites, while maintaining all the advantages of a passive system.

Briefiy, the invention is predicated upon the concept of an inter-coupled modified Van Atta antenna array which produces a pattern, substantially independent of vehicle attitude, in the shape of a hollow cone centered on the axis of the incident signal. The cone vertex angle is varied by varying the frequency of the incoming wave, thus giving high gain in the direction of the receiver, whose distance to the transmitter is determinative of the frequency employed.

The above mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will best be understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:

3,500,417 Patented Mar. 10, 1970 FIG. 1 illustrates a satellite embodying the antennarepeater of the invention;

FIG. 1a is a detail of an antenna element;

FIG. 2 is a schematic of a principal diagonal of the antenna;

FIG. 2a is a schematic illustration of a conventional lumped constant transmission line, the designations Z and Z representing reactive impedances; and

FIG. 3 illustrates the satellite-earth-radiation pattern relationship.

The invention is perhaps easiest understood by first visualizing a Van Atta array in two dimensions. The Van Atta array in one dimension is a collinear array of antenna elements in which each element is connected to an element a similar distance from the center; the interconnections being of equal electrical length. Consequently, a signal striking the array will be reflected with high gain as a pencil beam directly back at the source. In two dimensions, Van Atta arrays are disposed as diameters of a circle with the interconnections occurring between two elements on the same diagonal, equidistant from, and on opposite sides of the circle center. The result of a signal striking this array would be similar to that of the collinear array, except with high gain.

As a first major modification, assume the line length between coupled elements is varied by an amount A, proportional to the radial distance of the coupled elements to the array center. The reflected pencil beam now defocuses to a hollow cone, the vertex angle of which depends upon A. Whether A is a positive or negative quantity is immaterial; in one case, the cone vertex occurs at the antenna, in the other it occurs below the antenna as the cone first converges (crosses) and then diverges.

Given a repeater or antenna with the foregoing characteristic, a signal beamed at the antenna would be reflected at high gain to receiver sites lying along a line defined by the intersection of the cone with the earth.

The second major modification will be explained in connection with FIGS. 1 to 3. In FIG. 1 there may be seen a dumbbell structure, the lower portion 10 of which contains the repeater or antenna, and the upper portion 30 a counterbalance. The reason for the counterbalance and the interconnecting rod 40 will be explained later.

Confined within a suitable domed shaped housing 11 is the antenna or repeater which comprises a plurality of antenna elements 13, each of which may take the form of a conical horn shown in the detail of FIG. 1a.

While horns have been chosen in the depicted embodiment, it will be appreciated by those skilled in the art that any antenna element may be employed (even a dipole). However, since there would be no purpose in selecting an element whose pattern subtends a greater area than that of the earth itself (when one considers that the antenna is embodied in a satellite), an antenna element with inherent gain (i.e., a horn) is selected. This selection will add to the overall gain efiected by the composite structure.

The antenna elements are disposed along diagonals (for example, 12 and 12) in a circular pattern as shown. Since each element has a finite width, the resultant open areas towards the periphery of the defined circle may be filled with additional elements disposed along interrupted diameters, that is, diameters discontinuous at the center (such as 14 and 14').

FIG. 2 schematically illustrates one such diagonal. While a continuous diagonal has been chosen for purposes of illustration, it will be appreciated that discontinuous diameters would be similar except for the omitted center elements.

Elements equispaced from the center are connected by dispersive microwave transmission media (i.e., the phase shift is notlinearly proportionate to frequency).

The physical length of the interconnecting members (2025) is unequal by an amount A; where A is proportionate to the radial distance of the connected elements to the center. Thus, for example, element 20 would be of a length X; element 21 of a length X-A; element 22 of a length X-2 etc., where X is a constant and A is a f(radius). Because the interconnections are dispersive, the cone vertex an legis dependent upon the frequency of the incident wave. This is because antenna-elementconnecting-lines unequal in electrical length produce a conical pattern whose vertex angle depends upon this inequality (A). Since the inequality is variable, so is the angle of the cone. I have chosen to call arrangements based upon the foregoing principle steered cone retrofiective systems.

Dispersive lines are well known in the art and may, for example, take the form of lumped constant transmission lines or waveguides near cut off.

FIG. 3 illustrates an operative example. The satellite is located, for example, 20,000 miles from the earth. A transmitter at point A desires to communicate with the receivers at B and C. The transmitter site calculates the distance to B and C and determines the distinct frequencies which would expand the cone to a radius equal to B and C, respectively. Simultaneous transmissions may then take place to both. Concurrently, a transmitter at D might communicate with E. No interference will result even if B were the same distance from A as E is from D. Although the same frequency would be chosen by each transmitter, the respective cones would each center about the axis of the incident signal.

Stabilization of the satellite in space is not critical. The antenna need only point in the general direction of the earth so as to contain the earth within each elements own area of gain. Accordingly, in the embodiment shown, gravity gradient stabilization is employed. Gravity gradient stabilization relies upon the phenomenon that a dumbbell in space will assume the attitude where its axis is along an earth radius. This occurs because the inner mass must go faster to maintain its orbit than the outer one. Since the two masses are coupled by a rod 40 (which may be 100 ft. long), they are forced to travel at the same average speed. Thus, the inner mass is forced to go slower and seeks to return to earth, and the outer to go faster and escape into space. As a result, the rod is in tension and seeks an attitude in alignment with the earths radius. To reduce the pendulum moment which would otherwise occur, the rod shown should be flexible to introduce a damping factor. In order to ensure that the repeater section of the satellite is towards earth vis-avis the counter-mass and not vice versa, the rod must be extended with some degree of timing precision at a point when the vehicle bearing the satellite has achieved orbit and the antenna-repeater section is earth oriented.

As compared with an Echo sphere of 140 foot diameter, an equivalent return signal is obtained from a 2-foot diameter array using the steered-cone retrofiective principle. Comparison with an active satellite system is more diflcult because the active element is power limited rather than gain limited, and a number of system factors can thus affect their relative performance. In general, an active repeater will deliver larger received signal, but the steered retrofiector technique has jamming advantages and higher reliability.

A comparison will be made of the performance of an active repeater, a passive Echo-type satellite, and a steered-cone retrofiector in a typical sytsem environment. The assumed parameters are tabulated and some performance factors set forth below:

Operating frequency-X-band Orbit-1000 miles above surface, 2000 mile range RF power output (active)5 watts max.

For communication between a ground transmitter and a ground receiver via an isotropic reflector P Gt r e where:

G =transmitting antenna gain (above isotropic) G receiving antenna gain (above isotropic) A =eflective reflector area R =received power P =transmitted power A=wavelength R slant range transmitter to reflector R =slant range reflector to receiver This equation applies to an Echo sphere, where A=projected area of sphere. For a flat plate, or a retrofiector such as a Van Atta array, with transmitting and receiving directions normal to the plane of the reflector where:

G antenna gain of the reflector ir D /k D diameter For other angles of incidence, the geometry requires that the transmitter direction and receiver direction make equal angles with the normal to the plane of the reflector. The reduced projected areas of the reflecting plate, due to angles other than normal, results in reduced gain. For the case P. mama;

where 9=angle from normal (applies to both transmitting and receiving directions).

Equation 3 also applies to a flat retrofiective array, such as a Van Atta array, in which the reflected signal returns toward the direction of arrival, and the transmitter and receiver directions are the same, so that the cos 0 term in Equation 3 is applicable.

In calculating the behavior of a steered-cone retroflective array, the satellite may have different gains in the receiving and re-radiating directions and the angles to the transmitter and receiver may not be equal.

The receiving gain of the steered-cone retrofiector is derived from its area In the re-radiating direction the gain depends on the cone angle selected. For receiving sites near the g ound transmitter, the array may transmit a needle beam in the direction of the transmitter.

This is the highest possible gain for a given size array. For receivers at other ground distances from the transmitter, a different frequency is selected, steering the cone angle, and the re-radiated pattern becomes a hol ow cone centered on the direction of the arriving wave. The radial beamwidth (angular wall thickness) of the hollow cone is determined by the retrofiector diameter, and the circumference of the pattern (considered as an illuminated area on the earth) is established by the selected cone angle, so that the downward gain varies inversely as the first power of ground range between transmitting and receiving sites.

Table I shows calculated performance of an active repeater satellite communication system using satellites in a 1000 mile orbit operating at X-band with an RF power output of 5 watts. When the system is loaded with a reasonable traffic load, the round trip loss (two-way cos 0 path loss less antenna gains in the vehicle and amplification on the vehicle) is seen to be 2.76 db.

Table H shows performance of a steered cone retroflector system. As seen in the table, a steered cone retrodirective array of 5 foot radius, under optimum conditions, can equal the power level received on the ground from an active satellite system. In general, the signal return from a passive steered-cone system will be weaker than the corresponding active system, but is not subject to overload by heavy trafiic or jamming, and has high antenna directivity for protection against interference or jamming. In addition, the passive system has essentially unlimited operating life.

TABLE I Active satellite communication system X-band-1000 mi. altitude2000 mi. slant range (path loss 183 db) 5 watts-RF power in satelliteAntenna gain 7 db max.,

to cover visible earth Small user-5 kw., 4 foot dish, 5 db receiver noise Large use10 kw., 60 foot dish, 2 db receiver noise Jammer-50 kw., 60 foot dish With heavy trafiic (8-10 large users) or moderate trafiic plus jamming (four large users plus one jammer), the received signal level in the satellite will be 62 dbw, or 37 db above the level from a small user. Under these conditions, the signal retransmitted in response to a small user signal will be 37 db below the full +7 dbw satellite power output, and an additional 1 db loss of signal power will result from the effect of hard limiting.

Retransmitted signal level 30 dbw Downward gain, satellite antenna 7 db Gain, 4 foot ground antenna 40 db Path loss 183 Received level (small user receiver) from one small user transmitted signal 166 dbw Received level (small user receiver) from a large user (same assumed trafiic) +140 dbw Round trip loss (subtracting ground antenna gains and power) :283 db. This represents 366 db two-way path loss, less satellite antenna gains (14 db) and amplification in the satellite (99-30=69 db).

TABLE II Passive steered-cone satellite communication system X-band1000 mi. altitude2000 mi. slant range (183 db path loss) Small userS kw., 4 foot dish Large user-10 kw., 60 foot dish (no restrictions necessary on jammer power, or total tralfic power).

6 where: A physical area of retroflective array 2 zfi T =path loss=183 db Power received by small user from a small user- P.= +37 dbw) 40 db+40 db)+10 log 20 log [i -366 db To equal the power received from an active satellite (see Table I).

166=187+20 log A A =12 sq. ft.

A steered retroflector with an area of 12 sq. ft. (2 foot radius) will provide a level at the ground receivers equal to the performance calculated for a 5 watt active satellite. The round trip loss (366 db two-way path loss, less 48 db antenna gain each direction) is 283 db for the most favorable case, with short range between the transmitting and receiving sites.

The cone pattern, being frequency steered, provides an unusual form of protection both against jamming and interference. Since the ground range between a transmitter and a receiver determines the required cone angle, which in turn is selected by choice of operating frequency, it follows that all signals received at a ground station falling in a given frequency hand must necessarily come from transmitters at a given ground range from the receiving point. A jammer, in order to jam a given receiver, must use the appropriate frequency band for his distance from the receiver, and is therefore unable to jam any other frequencies. Similarly, classes of traflic are separated. Short-distance traffic between groups of small users in one area, or to larger stations in the same local area, falls in a definite frequency band giving high satellite gain (needle-beam return pattern) and this frequency band is not subject to jamming, overload, or interference by distant stations. Similarly, communication at other distances is segregated in frequency bands which are not subject to interference by stations at any other range from the receiving station.

In each local area, the short range frequency provides party line communication between small users, and communication with a local center. This communication does not interfere with similar communications in the same band in another area, they are not subject to jamming from outside the local area, and they will be difiicult to intercept from outside the local area.

Larger users, probably including network centers of the local areas, will operate in other frequency bands for point-topoint communications up to several thousand miles. These users do not interfere with the weaker, shorthaul trafiic, since the passive repeater is not subject to overload or limiting, and the operating frequencies differ. The round trip loss via the satellite is greater (due to larger cone angles) but this loss applies to all users and jammers. Jamming is only possible when the jammer and the friendly transmitter are equally distant from the receiver location. Interception also requires a receiver at the appropriate distance from the transmitter. In the event of jamming, a particular jammer can be evaded completely (approximately 40 db antenna directivity) by relaying traffic through a station at some other ground distance. This capability appears to be convenient in any large network of stations, and could also be used against accidental interference.

While I have described above the principles of my invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention as set forth in the objects thereof and in the accompanying claims.

I claim:

1. A retrofiective antenna comprising a plurality of antenna elements arranged in a collinear array, and a dispersive transmission media connecting each element in the array to a element equidistant from the array center, the media length being varied by an amount proportional to the distance between coupled elements.

2. The retroflective antenna claimed in claim 1 redundant in two dimensions with the collinear arrays arranged as diameters crossing at their respective centers.

3. A satellite repeater comprising the antenna claimed in claim 2, and means for generally orienting said antenna toward earth.

4. A satellite including a passive repeater comprising a plurality of collinear arrays of antenna elements disposed as diameters of a circle, dispersive transmission media connecting each element to another element along the diameter equidistant from the array center, the media lengths being varied by an amount proportional to the distance between coupled elements.

5. The satellite repeater claimed in claim 4 in which the dispersive media is a waveguide near cutoff.

6. The satellite repeater claimed in claim 4 in which the dispersive media is a lumped constant transmission line.

7. The satellite claimed in claim 4 further comprising means for generally orienting said repeater towards earth. 8. The satellite repeater claimed in claim 7 in which said orienting means comprises a dumbbell structure, said repeater forming one of the two dumbbell masses.

References Cited UNITED STATES PATENTS Proceedings of the IRE, June 1961, pp. 1066-1074.

ELI LIEBERMAN, Primary Examiner US. Cl. X.R. 

